The invention relates in general to the operation of a refrigeration system, and more specifically to the control of at least one electronic expansion valve coupled to an economizer in a refrigeration system.
Refrigeration systems generally include a refrigerant circuit including a compressor, a condenser, a main throttling device, and an evaporator. Vapor refrigerant is delivered to the compressor where the temperature and pressure of the vapor refrigerant is increased. The compressed vapor refrigerant is then delivered to the condenser where heat is removed from the vapor refrigerant in order to condense the vapor refrigerant into liquid form. The liquid refrigerant is then delivered from the condenser to a main throttling device, such as a mechanical thermostatic expansion valve. The main throttling device restricts the flow of the liquid refrigerant by forcing the liquid through a small orifice in order to decrease the pressure of the liquid and therefore decrease the boiling point of the liquid. Upon exiting the main throttling device, the liquid refrigerant is in the form of liquid refrigerant droplets. The liquid refrigerant droplets are delivered from the main throttling device to the evaporator, which is located within or in thermal communication with the space to be conditioned by the refrigeration system. As air passes over the evaporator, the liquid refrigerant droplets absorb heat from the air in order to cool the air. The cooled air is circulated through the conditioned space to cool the masses within the conditioned space. Once the liquid refrigerant droplets have absorbed sufficient heat, the liquid refrigerant droplets vaporize. To complete the refrigeration cycle, the vapor refrigerant is delivered from the evaporator back to the compressor.
An additional heat exchanger in the form of an economizer may be added to the refrigeration system in order to enhance the efficiency of the cycle. The economizer is often coupled between the condenser and the main throttling device. Specifically, the economizer is coupled to the condenser by an economizer input line having a first branch and a second branch. The first branch delivers refrigerant through the economizer to the main throttling device. The second branch delivers refrigerant through a secondary throttling device, through an economizer chamber within the economizer, and back to the compressor. In an economizer system, the refrigerant flowing to the main throttling device is routed through the economizer to be sub-cooled, while some refrigerant is drawn off through the second branch of the economizer input line to a secondary throttling device. The drawn-off refrigerant passes through the secondary throttling device, where it is cooled by the throttling process, and into the economizer chamber. Once in the economizer chamber, the drawn-off refrigerant is in a heat transfer relationship with the refrigerant flowing through the first branch of the economizer input line to the main throttling device. The drawn-off refrigerant absorbs heat from the refrigerant flowing through the first branch to the main throttling device. Thus, the refrigerant flowing through the first branch is sub-cooled. Liquid refrigerant is sub-cooled when the temperature of the liquid is lower than the vaporization temperature for the refrigerant at a given pressure. The drawn-off refrigerant absorbs heat until it vaporizes.
Before the drawn-off refrigerant is directed back to the compressor, the vaporized refrigerant has generally reached a superheat level. The refrigerant reaches a superheat level when all of the refrigerant has vaporized and the temperature of the refrigerant is above the vaporization temperature for the refrigerant at a given pressure. The refrigerant at the superheated level is then directed back to the compressor.
The operating conditions of the refrigeration system are controlled, in part, by the operation of the economizer. The economizer is controlled by the secondary throttling device. Generally, the main and secondary throttling devices are mechanical thermostatic expansion valves (TXV), which operate based on the temperature and pressure of the refrigerant passing through the valve.
The use of TXVs for the main and secondary throttling devices has several limitations. First, TXVs cannot be dynamically adjusted to control the operating conditions of the refrigeration system. TXVs are initially designed to optimize the operating conditions of the refrigeration system, but the TXVs cannot be dynamically adjusted to optimize the operating conditions at all times.
Moreover, TXVs can only accommodate one set of operating conditions. A TXV in the economizer cycle is generally designed to maintain one set of primary operating conditions. However, extraordinary or secondary operating conditions may occur, which may demand the primary operating conditions to be overridden. A TXV cannot accommodate secondary operating conditions that may be desired to periodically override the primary operating conditions.
Accordingly, the invention provides a method and apparatus for controlling at least one electronic expansion valve coupled to an economizer in a refrigeration system in order to dynamically control the refrigeration system operating conditions and in order to accommodate more than one set of operating conditions. The refrigeration system generally includes a compressor, a condenser coupled to the compressor, a heat exchanger coupled to both the condenser and the compressor, an evaporator coupled to both the heat exchanger and the compressor, and an electronic expansion valve, an input of the valve coupled between the condenser and the heat exchanger, an output of the valve coupled to the compressor.
The above-described structure is normally operated under a set of primary operating conditions. One condition is that the state of the refrigerant flowing from the heat exchanger to the compressor is maintained above superheat temperature. More specifically, the pressure between the heat exchanger and the compressor is sensed. The sensed pressure is converted into a saturation temperature value. The temperature between the heat exchanger and the compressor is sensed. The saturated suction temperature value is compared to the sensed temperature. The flow of refrigerant through the EXV is adjusted until the sensed temperature is greater than the saturated suction temperature value.
The above-described structure can also be used to control the capacity of the system. More specifically, the method can include determining the required capacity of the system and adjusting the flow of refrigerant through the electronic expansion valve to adjust the actual capacity of the system toward the required capacity of the system. For example, if the required capacity is less than the actual capacity, then the flow of refrigerant through the electronic expansion valve can be decreased. Likewise, if the required capacity is greater than the actual capacity, then the flow of refrigerant through the electronic expansion valve can be increased. In either event the method may require that the primary set of operating conditions be overridden.
In another aspect of the invention, the system is operated to maintain the power of the system below a threshold value (e.g., below a max rated horsepower). This method includes determining the power required to operate the compressor based on the measurement of system parameters; comparing the power required to a threshold value; and adjusting the flow of refrigerant through the electronic expansion valve in order to keep the power required below the threshold value. There are many different ways to determine the required power (e.g., by sensing the pressure between the heat exchanger and the compressor, the pressure between the evaporator and the compressor, the pressure between the compressor and the condenser, and the flow rate of refrigerant). In this embodiment, if the horsepower required to operate the compressor is less than the threshold value, then there is no need to adjust the flow of refrigerant through the electronic expansion valve. However, if the power required to operate the compressor is greater than the threshold value, then the flow of refrigerant through the electronic expansion valve can be decreased to avoid operating the system above its rated power limit. In order to do this, the primary operating conditions may need to be overridden.
In yet another aspect of the invention, the system is operated in order to prevent overheating of the compressor. More specifically, the flow of refrigerant from the heat exchanger to the compressor can be adjusted so that some amount of liquid refrigerant is provided to quench the compressor. The method includes measuring a system parameter corresponding with the temperature of the compressor; comparing the measured system parameter to a threshold value; and adjusting the flow of refrigerant into the compressor by adjusting the flow of refrigerant through the electronic expansion valve in order to keep the system parameter below the threshold value. The system parameter can be any parameter that corresponds with the temperature of the compressor (e.g., the temperature of the compressor, the temperature of refrigerant flowing from the compressor, etc.). In practice, if the system parameter exceeds the threshold value, the flow of refrigerant through the electronic expansion valve can be increased in order to provide a volume of liquid refrigerant to quench the compressor. In order to do this, the primary operating conditions may need to be overridden.
Other features and advantages of the invention will become apparent to those of ordinary skill in the art upon review of the following description, claims, and drawings.